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INA326EA250G4 Datasheet(PDF) 11 Page - Texas Instruments |
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INA326EA250G4 Datasheet(HTML) 11 Page - Texas Instruments |
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11 / 23 page  INA326, INA327 11 SBOS222D www.ti.com Applications sensitive to the spectral characteristics of high- frequency noise may require consideration of the spurious frequencies generated by internal clocking circuitry. “Spurs” occur at approximately 90kHz and its harmonics (see typical characteristic “Input-Referred Ripple Spectrum”) which may be reduced by additional filtering below 1kHz. Insufficient filtering at pin 5 can cause nonlinearity with large output voltage swings (very near the supply rails). Noise must be sufficiently filtered at pin 5 so that noise peaks do not “hit the rail” and change the average value of the signal. Figure 3 shows guidelines for filter cutoff frequency. HIGH-FREQUENCY NOISE C2 and CO form filters to reduce internally generated auto- correction circuitry noise. Filter frequencies can be chosen to optimize the trade-off between noise and frequency re- sponse of the application, as shown in Figure 3. The cutoff frequencies of the filters are generally set to the same frequency. Figure 3 shows the typical output noise for four gains as a function of the –3dB cutoff frequency of each filter response. Small signals may exhibit the addition of internally generated auto-correction circuitry noise at the output. This noise, combined with broadband noise, becomes most evi- dent in higher gains with filters of wider bandwidth. INPUT BIAS CURRENT RETURN PATH The input impedance of the INA326 is extremely high— approximately 1010 Ω. However, a path must be provided for the input bias current of both inputs. This input bias current is approximately ±0.2nA. High input impedance means that this input bias current changes very little with varying input voltage. Input circuitry must provide a path for this input bias current for proper operation. Figure 4 shows provision for an input bias current path in a thermocouple application. Without a bias current path, the inputs will float to an undefined poten- tial and the output voltage may not be valid. INPUT COMMON-MODE RANGE Common instrumentation amplifiers do not respond linearly with common-mode signals near the power-supply rails, even if “rail- to-rail” op amps are used. The INA326 uses a unique topology to achieve true rail-to-rail input behavior (see Figure 5, “Inside the INA326”). The linear input voltage range of each input terminal extends to 20mV below the negative rail, and 100mV above the positive rail. INPUT PROTECTION The inputs of the INA326 are protected with internal diodes connected to the power-supply rails. These diodes will clamp the applied signal to prevent it from damaging the input circuitry. If the input signal voltage can exceed the power supplies by more than 0.5V, the input signal current should be limited to less than 10mA to protect the internal clamp diodes. This can generally be done with a series input resistor. Some signal sources are inherently current-limited and do not require limiting resistors. FILTERING Filtering can be adjusted through selection of R2C2 and ROCO for the desired trade-off of noise and bandwidth. Adjustment of these components will result in more or less ripple due to auto-correction circuitry noise and will also affect broadband noise. Filtering limits slew rate, settling time, and output overload recovery time. It is generally desirable to keep the resistance of RO relatively low to avoid DC gain error created by the subsequent stage loading. This may result in relatively high values for CO to produce the desired filter response. The impedance of ROCO can be scaled higher to produce smaller capacitor values if the load impedance is very high. Certain capacitor types greater than 0.1 µF may have dielec- tric absorption effects that can significantly increase settling time in high-accuracy applications (settling to 0.01%). Polypro- pylene, polystyrene, and polycarbonate types are generally good. Certain “high-K” ceramic types may produce slow settling “tails.” Settling time to 0.1% is not generally affected by high-K ceramic capacitors. Electrolytic types are not recommended for C2 and CO. INA326 Thermocouple 5 FIGURE 4. Providing Input Bias Current Return Path. |
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